The invention relates to a novel method and apparatus for viewing the molten pool of metal in an arc welding process.
The process of arc welding is one of the most widely used manufacturing processes in the world. Efforts to automate the various aspects of the welding process have been especially great in recent years. The efforts have centered on attempting to automate the welding process to allow the use of robots which would be capable of producing very uniform, high quality welds. The reasons for seeking such automation are twofold: first, to increase productivity by increasing the speed and accuracy of the welding process in routine welding applications; and secondly, to increase the quality of the welds thus produced allowing automated welding to be utilized in high quality control industries such as the nuclear power industry.
In order to appropriately control the arc welding process and thereby allow for practical automation of the processes, numerous attempts have been made to objectively measure and discern precisely the pertinent parameters in the welding process. It has been said in the past that only the welding operator truly knows the quality of the weld and what is going on in the welding process. This is true because only he in the past has been able see precisely the quality and physical parameters of weld pool size and arc length. Also, only the operator can truly track the often irregular weld preparations or joints.
Attempts have been made to track welds by use of sensors which look ahead of the welding area. Such sensors attempt to discern where the weld preparation area or joint is and adjust the movement of the welding torch so as to coincide with the weld preparation or joint area. Such methods have utilized both direct contacting type sensors which are dragged through the weld preparation area ahead of the welding torch and by indirect measurement techniques, such as the use of infra-red detectors.
In recent years, use of television direct monitoring has increased. All of the methods of directly monitoring the arc welding area with a television camera have involved attempting to duplicate what the welding operator actually sees. Thus, the camera has been placed in such a position as to have an oblique view of the weld similar to the view enjoyed by the operator. Such an oblique view of the weld has the inherent problem of also viewing the arc that is very bright which tends to wash out the entire picture unless appropriate filters are utilized. Therefore, the methods of obliquely viewing the weld and arc area have also involved filtering the view by the camera to one or several discrete wave lengths of light, rather than viewing the broad spectrum of light that is available from the arc.
It has also been known for some time that there is a relationship between the actual width of the molten pool and the penetration being achieved by the welding process. Therefore, it has been thought to be desirable to be able to directly measure and also to control weld pool width. Precise control of the weld puddle width produces a correspondingly precise control of the penetration being achieved by the welding process.
Prior methods of analysis of video data being received from viewing the arc and weld pool area have made the assumption that the bright areas represent the weld pool and that once the light intensity has decreased to a certain value, then the edge of the weld pool has been approached. Such methods then employ a go/no-go type of binary logic system to establish the weld pool width. Unfortunately, there are oscillations to the weld pool caused by the fluctuation in voltage of the arc and also caused by the motion of the electrode forward along the weld preparation area. Such oscillatory motions cause the area of brightness to look considerably larger than it actually is. Attempts to mitigate the data error caused by the oscillatory motions have been precisely that--simply corrections of the data rather than actual measurement and evaluation of the true weld pool edge.
Thus, the problems of obliquely viewing the weld area with a television monitoring camera are that unforeseen obstructions of the camera in real welding situations can arise. Such obstructions can be caused by the weld preparation geometry and by constraints on the placement of the camera. Also, distortion of the image of the weld puddle is caused by the oblique viewing system. These are in addition to the over-exposure problems caused by direct arc viewing which are reviewed above.
It has been anticipated for some time that the best opportunity to provide reliable control to the welding process involves adequately and accurately measuring the process parameters at the point of welding. Welding parameters that can be measured directly at the point of welding seam tracking are voltage and current. The primary emphasis in this area has been tracking of weld seams or weld preparation areas. Unfortunately, in order to avoid damage to the sensor and also to keep the sensor from being obscured by the arc area and the melted metal, it has been necessary to sense the track some distance, even small, ahead of the weld torch. This immediately produces the requirement of having some delay in system response so that in fact the system responds to seam tracking changes at the time when the welding head is over the area of change.
Seam tracking devices have been of two types. First are those that use the arc itself as the sensor. Sensors of this type sense voltage and current variations in the arc when various surface features of the base metal are encountered. One such method oscillates the arc back and forth across the seam noting the voltage changes as the arc gets longer as it approaches the weld preparation. Various schemes have been proposed to allow for this oscillation. Both magnetic and mechanical motion devices have been utilized previously. The second method is the direct arc viewing method for seam tracking directly at the point of welding. Here the methods identify the edges of the weld preparation or weld groove from analysis of light from the arc reflected off the edges of the grooves or the side walls of the weld preparation area. A feedback system is then provided to respond to the reflected light to produce the desired effect of following the seam. Additionally, the problems of tracking the weld seam or weld preparation area by viewing the area ahead of the weld require that any information thus received be delayed before it is implemented so that the welding torch is indeed over the area detected, or the change detected, at the time the change instruction or signal is initiated. Also, any process control data whether it be used for weld seam tracking or for viewing welding parameters in the area of the arc and molten pool, are subject to error due to parallax from the oblique view of camera positions known in the prior art, and the undesirable masking of the far side of the weld pool by the arc itself.